Two phases typically occur during an upshift of an automatic transmission of an automatic vehicle drive line, namely, a torque phase and an inertia phase. Before the upshift is commanded, the torque input to the transmission clutches includes only the engine combustion torque passing through a torque converter. During the torque transfer phase, pressure is applied to the incoming clutch element. The input torque is split between the outgoing clutch and the incoming clutch. By the end of the torque phase, the torque carried by the outgoing clutch drops to zero and all the torque is carried by the incoming clutch element. The output torque is reduced because of the change in gear ratio.
After the torque transfer is completed, the ratio transfer phase or the inertia phase begins. During this phase, as the pressure on the oncoming element increases further, the engine speed is reduced rapidly to the level of the new gear ratio. This creates a large inertia torque which has to be absorbed in addition to the combustion torque by the clutch element. This inertia torque creates a torque bump which is transmitted to the passenger compartment. The magnitude of the torque bump is dependent on the clutch pressure at the start of the inertia phase.
One method of reducing the inertia bump is to reduce the pressure to the clutch during the shift. However, this lengthens the shift time (torque phase and the inertia phase) and, consequently, the shift feels dragged out. Also, a large amount of heat is created which has to be absorbed by the clutches.
A number of U.S. patents disclose shift shock minimizing systems which control the line pressure supplied to the on-coming transmission friction element. For example, the U.S. patent to Ishimaru et al., U.S. Pat. No. 3,855,880 discloses a system which reduces line pressure to the on-coming transmission friction element just prior to engagement of that element in order to provide a smooth coupling.
In the U.S. patent to Burkel et al., U.S. Pat. No. 4,611,507, turbine speed is sensed and inputted into an ECU. This sensed speed signal is used to determine the engagement pressure supplied to the on-coming transmission friction element.
Other U.S. patents discuss the torque and inertia phases associated with shifts. For example, the U.S. patents to Downs et al., U.S. Pat. Nos. 4,653,350 and 4,707,789 disclose how output torque is developed during a shifting operation. FIG. 2B of each of these patents represents the pressure supplied to the on-coming clutch during the shifting operation. In both of these patents, the pressure to the on-coming clutch is initially boosted during a fill phase, reduced during a torque phase and ramped during an inertia phase.
U.S. Pat. No. 4,845,618 issued to Narita discloses a system for controlling clutch pressure during a shift. The clutch pressure is maintained at a stored target value that is called out as a result of the monitoring of turbine torque. The line pressure supplied to the oncoming clutch is determined based on sensed engine output and torque converter output speeds.
A number of U.S. patents disclose control systems for minimizing shocks during shifting operations in transmissions wherein the engine spark timing is adjusted in order to reduce output torque during the shifts.
For example, in the U.S. patent to Mori U.S. Pat. No. 4,680,988, the engine spark timing is employed to reduce output torque during shifting. The supply pressure to the on-coming transmission friction element during the shift is dependent upon a sensed ratio of input-to-output speed of the transmission The '988 patent further describes a closed loop control for a clutch during a shift using a continuous monitoring of speed ratio across the clutch.
In the U.S. patent to Takeda et al., U.S. Pat. No. 4,744,031, the ignition timing is retarded to reduce output engine torque after a specified time delay from the occurrence of a shift command and prior to unlocking of the torque converter.
In the U.S. patent to Hrouat et al., U.S. Pat. No. 4,792,902, there is disclosed a system which controls the ignition timing of an internal combustion engine in accordance with the magnitude of engine speed and transmission output speed. The spark timing is altered as a function of the inertia torque.
Finally, in the U.S. patent to Yasue et al., U.S. Pat. No. 4,800,781, spark ignition timing is retarded prior to shifting and the fuel supplied to the internal combustion engine is increased immediately after the shifting operation.